1. Field of the Invention
This invention relates to a magneto-optical recording medium such as magneto-optical disk and card, a manufacturing method for the medium and a magneto-optical recording and playback device to record and play back data using the magneto-optical recording medium. This invention specially relates to a magneto-optical recording medium having a recording layer comprising a garnet ferrite layer and being ideal for high density and proximity recording, a manufacturing method for the medium and a magneto-optical recording and playback device for the medium.
2. Background Art
In conventional magneto-optical recording media on the market, the recording layer is mainly made from a thin metal layer. Data are recorded onto the recording layer as recorded bits by changing the optical properties such as the transmission or reflection rate of minute spots on the recording layer by a light beam for recording data. An amorphous alloy of rare earth metals such as TbFeCo is a typical material for the thin metal layer, and the alloy is favorable for recording data because it generally has a high absorption coefficient ( greater than 105 cmxe2x88x921). However, the thin metal layer is prone to deterioration due to oxidation and other factors. For this reason, it has to be sealed and protected by a plastic layer, for example.
On the other hand, a magneto-optical recording medium having a recording layer that consists of an oxide such as garnet ferrite, which is a kind of ferrite having a garnet type structure of crystals and has a large Faraday rotation angle, has been developed. In such a recording medium, the degree of deterioration of properties due to oxidation is smaller than that of the case in which a metal material is used for the recording layer, because the material itself of the recording layer is already an oxide. Therefore, it has the distinctive feature that the above mentioned special protection is not necessary.
In the case in which garnet ferrite is used as the material for magneto-optical recording, however, internal stress occurs in a garnet ferrite layer when spattering for formation of the layer on a substrate. This sometimes results in cracks in the garnet ferrite layer, rough morphology of the surface of the layer and very large crystal particles, which are not preferable because they cause the problem of medium noise when recording and playing back. In order to overcome the above-mentioned problems, a method for improvement of the morphology of the layer by adjusting the thermal expansion coefficient of the substrate and by performing inverse spattering, for example, after annealing, is disclosed in Japanese Patent Publication No. Hei 8-249740 (1996).
Meanwhile, a new type of recording layer having a plurality of layers made from several kinds of materials for magneto-optical recording has recently been developed for the purpose of improvement of the S/N and C/N ratio. However, it is said that metal materials such as the above amorphous alloy are not suitable for multi-layered structures since they have a relatively small Kerr Effect and a high absorption coefficient. As a result, a recording layer having multiple-layers made from garnet ferrite, which is known as an oxide-type material for optical recording and which has large Faraday Effect, has attracted much attention. For instance, a multi-layered recording medium having piled layers made from bismuth-substituted type garnet ferrite having an excellent Faraday rotation angle in the wavelength range of visible light has been proposed (see Itoh, Koike, Numata, Inoue and Kawanishi xe2x80x9cMulti-Layered Magnetic Garnet Ferrite Film for Magneto-Optical Recordingxe2x80x9d, Abstract of 10th Academic Lectures for Application of Magnetics in Japan, p.31, Nov. 1986).
However, a multi-layered recording medium having a recording layer made from bismuth-substituted garnet ferrite requires a high power light beam when writing data because the absorption coefficient of the recording layer is small. Taking this problem into consideration, Japanese Patent Publication No. Hei 6-282868 discloses a multi-layered type magneto-optical recording medium having a light absorption layer which accelerates the recording process by efficient transfer of the applied by a light beam to a recording layer which is near the light absorption layer. However, the above mentioned magneto-optical recording medium uses direct energy gap semiconductors such as GaAs, InP, CdS, CdSe, ZnSe and ZnS, which are easily oxidized during layer formation, as the light absorption layer. Therefore, a protective layer is required on the surface of the light absorption layer. The protective layer is formed by deposition of SiO2, Al2O3, TiO2 or the like in the range of 2xcx9c100 xcexcm by a CVD method or spattering to create a film. Accordingly, the manufacturing process shown in Japanese Patent Publication No. 6-282868 becomes complex because formation of the light absorption layer and the protective layer is necessary in addition to formation of the recording layer, which leads to increased production costs. Besides, properties such as the S/N ratio of the magneto-optical recording medium manufactured by the above method have not yet reached a satisfactory level.
Japanese Patent Publication No. Hei 6-290497 (1994) discloses a manufacturing method for a magneto-optical recording medium having a recording layer that has a double-layered garnet ferrite structure, in which a non-magnetic garnet ferrite underlayer is used, and discloses that the multi-recording layer keeps the garnet ferrite crystal particle diameter at 1 xcexcm or below to reduce the disparities in bit shapes and medium noise. However, the above method is impractical because the manufacturing process is complex. Further, in the case in which the multi-layered structure is formed by two types of garnet ferrite having different compositions, the elements in each garnet ferrite layer disperse at the vicinity of layer boundaries after heat treatment. Therefore, compositional deviations occur in the direction perpendicular to the layer surface in the multi-layered structure, which causes the problem of deterioration of properties such as the S/N ratio and repeatability. Furthermore, high density recording is hindered if the above mentioned method is employed because it is impossible for the above mentioned garnet ferrite layer formation method to provide minute crystals on the order of nanometers.
On the other hand, various approaches to the improvement of the S/N and C/N ratios have been examined from not only the aspect of the magneto-optical recording medium itself but also from the aspect of devices for recording and playing back data on the magneto-optical recording medium.
The method to realize high resolving power by raising the optical refraction rate by fulfilling a liquid between a sample and an object lens is known. An application of this method using a small solid lens has been proposed under the name of SIL lens (Appl. Phys. Lett., 57(24), 1990; U.S. Pat. No. 5,004,307). And, a data recording and playback system using the SIL lens for magneto-optical recording medium has been also proposed (U.S. Pat. No. 5,125,750). This system is characterized in that the distance between the SIL lens and the recording layer of the recording medium is kept within the wavelength of the light, namely on the sub-micron order, so as to gain a small focus spot diameter which is a feature of the SIL lens, and so as to gain an increase in the recording density. However, even in this system, the S/N and C/N ratios have not yet reached a satisfactory level. Besides, this system lacks generality because it is not suitable for a magneto-optical recording medium that has a relatively thick protective layer, which is, for example, disclosed in Japanese Patent Publication No. Hei 6-282868.
The present invention was made referring to the state of the prior art mentioned above. The present invention aims to provide data recording and playback with excellent S/N and C/N ratios, from the both aspects of a magneto-optical recording medium and a magneto-optical recording and playback device. Namely, it is an object of the present invention to produce a new magneto-optical recording medium that has a recording layer containing a garnet ferrite layer, which has high resolution, high recording density, high S/N and C/N ratios and the magnet properties of which can be easily controlled.
It is another object of the present invention to produce a new magneto-optical recording and playback device that is suitable for data recording and playback on such a magneto-optical recording medium and is able to increase the S/N and C/N ratios.
The above mentioned object is achieved by a magneto-optical recording medium having a recording layer and a reflective layer on a substrate characterized in that the recording layer has a layered structure in which a garnet ferrite layer and any one of a spinel ferrite layer, a rutile-type oxide layer and a hematite layer are layered. The layered structure in the recording layer is preferably present at least on tracks in which data are recorded. If the layered structure is not present between the tracks, it is preferable that only a garnet ferrite layer be present between the tracks.
The recording layer may be located between the substrate and the reflective layer. On the other hand, the reflective layer may be located between the substrate and the recording layer. The preferable thickness of the garnet ferrite layer is 40 to 400 nm, and that of the spinel ferrite layer, the rutile-type oxide layer and the hematite layer is 10 to 100 nm. The recording layer may have a multi-layer structure in which a plurality of garnet ferrite layers and a plurality of spinel ferrite layers, rutile-type oxide layers and hematite layers are layered. If so, the preferable thickness of the recording layer is 40 to 100 nm. Further, grooves may be formed on, or loads may be attached to the surface of at least one of substrate, the reflective layer or the recording layer. Here, a xe2x80x9cloadxe2x80x9d is a member to change the effective reflection index on the surface of the layers, which has a rectangular section in general and forms convexities on the surface of the layers to which it is attached. The material of the load is not restricted.
In the magneto-optical recording medium of the present invention, a transparent layer may be formed on the surface of the recording layer or the reflective layer. If so, grooves may be formed on the surface of the transparent layer.
The magneto-optical recording medium of the present invention can be produced by a manufacturing method characterized by comprising a step of heat treatment at a temperature of 500 to 700xc2x0 C., preferably 600 to 630xc2x0 C., after the formation of the recording layer.
The other object of the present invention mentioned above is achieved by a magneto-optical recording and playback device to record and play back data by use of a magneto-optical recording medium, characterized in that the wavelength of light is different for recording data into the magneto-optical recording medium and for reading data from the magneto-optical recording medium. This magneto-optical recording and playback device can be used for a magneto-optical recording medium having a recording layer comprising a garnet ferrite layer, preferably for a magneto-optical recording medium having a recording layer and a reflective layer on a substrate, and the recording layer having a layered structure in which a garnet ferrite layer and any one of a spinel ferrite layer, a rutile-type oxide layer and a hematite layer are layered. It is preferable that the light for recording and reading are provided by one light source.
This invention has the effect that a garnet ferrite-type recording medium having excellent magnetic properties and suitable as a magneto-optical recording medium can be produced without a complicated process. And, passivation does not occur since no metal-type material is used as the magneto-optical recording material.
Besides, a magneto-optical recording medium with high anisotropy, high resolution, high recording density and low noise can be produced since a garnet ferrite layer having minute morphology suitable for high density recording can be obtained, and internal stress in a recording layer can be cancelled by a combination of a spinel ferrite layer, a rutile-type oxide layer or a hematite layer with the garnet ferrite layer. Further, the control of the magnetic properties of the garnet ferrite layer is easily accomplished. Further, the S/N ratio of the recording medium is remarkably improved because of the synergistic effect of high output effect derived from the enormous Faraday effect originally provided by the garnet ferrite and the low noise effect.
In the case in which the reflective layer is located between the substrate and the recording layer, the production process of the magneto-optical recording medium can be simplified and the production costs can be reduced since a protective film is not necessary even if the reflective layer is made of metal-type material. Further, in this case, an optical pickup mechanism such as a read head can be set substantially closer to the medium surface during playback since there is no protective film. This makes it possible to obtain a higher S/N ratio than before.
In the case in which the recording layer comprises a plurality of garnet ferrite layers and a plurality of spinel ferrite layers, rutile-type layers or hematite layers, the number of heat treatments can be reduced. And, it is easy to obtain a recording layer having excellent magnetic properties since precise control of the stress in the recording medium is possible.
Further in the case in which grooves are formed, or loads are attached to the surface of at least one of the substrate, the reflective layer or the recording layer, servo control of the recording position on the recording medium can be carried out.
Further, if a transparent layer is layered on the surface of the recording layer or the reflective layer, the light irradiated from a playback head and focused on the recording layer will not be subject to the effects of dust and scratches on the surface of the recording medium. Incidentally, if grooves are formed on the transparent layer, the grooves can be utilized as guides for the servo control.
According to the manufacturing method of the magneto-optical recording medium of the present invention, it is possible to endow with magnetic properties only to the garnet ferrite layer present on the track parts, and to make the garnet ferrite layer except for the track parts non-magnetic by control of the temperature of the heat treatment. Therefore, it is possible to reduce the noise derived from parts other than the tracks, and the S/N ratio increases considerably because of the synergistic effect of the high output derived from the enormous Faraday effect originally provided by the garnet ferrite and the noise reduction effect. Further, it is also possible to reduce the magnetic interference with the data recorded in the track parts from the parts other than the track parts.
The manufacturing method of the present invention can produce a magneto-optical recording medium having a layered structure in which a garnet ferrite layer and any one of a spinel ferrite layer, a rutile-type oxide layer and a hematite layer are layered, at least on tracks in which data are recorded, as well as the other magneto-optical recording media of the present invention. And, in the case in which a reflective layer is located between a substrate and a recording layer, no protective means such as a coating layer for the reflective layer is necessary even if the reflective layer is made of metal type materials. Therefore, it is possible to simplify the manufacturing process and to reduce the production cost. Furthermore, it is possible to locate a light pick-up mechanism such as a reading head substantially closer to the recording layer, which results in a increase of the S/N ratio.
If the magneto-optical recording medium of the present invention has a transparent layer other than the substrate, compatibility with a conventional medium can be obtained, and a light beam focused on the recording layer will be less affected by dust that may be adhering to the surface of the recording medium or by scratches that may be present thereon. If grooves are formed on the surface of the transparent layer, servo control of the recording position becomes possible by detecting the change of the refraction and reflection rate on the surface of the transparent layer, which is caused by the grooves.
Taking the light absorption properties of a magneto-optical recording medium to be used into consideration, in the magneto-optical recording and playback device of the present invention, the wavelength of light is set to be different for recording data and reading data. Therefore, the S/N ratio and C/N ratio can be increased by optimizing the recording and playback, referring to the properties of the medium to be used.
For example, if the light absorption rate of the magneto-optical recording medium is high for light with a short wavelength, efficient data recording and reduction of the power of the light beam for data recording are possible by shortening the wavelength of the light beam for data recording. On the other hand, if the light absorption rate of the magneto-optical recording medium is low for light with a long wavelength, reducing undesirable heating of a recording layer is possible by lengthening the wavelength of the light beam for data reading, and increasing of C/N ratio is possible because a reflected light beam with high power can be obtained.
The magneto-optical recording and playback device of the present invention can be preferably used for the magneto-optical recording medium of the present invention that has a garnet ferrite layer.